1. Field of the Invention
The present invention relates to the forming of a Schottky diode.
2. Discussion of the Related Art
In the field of semiconductor components, the material which is currently mainly used is silicon. For very high breakdown voltages, silicon carbide is a priori preferable since silicon carbide can provide breakdown voltages per thickness unit approximately 10 times greater than silicon.
However, in the present state of technology, the methods currently used for the manufacturing of silicon-based components cannot be used for the manufacturing of components based on silicon carbide (SiC). In particular, it is presently impossible, in practice, to perform implantations and diffusions of P-type dopants into N-type doped silicon carbide, noting that the P-type dopant currently used for silicon carbide is aluminum and the N-type dopant is nitrogen. Indeed, a diffusion anneal for a P-type dopant would require temperatures on the order of 1700xc2x0 C., which raises acute technological problems.
The elementary structure of a Schottky diode is illustrated in FIG. 1. This diode is formed from a heavily-doped N-type substrate 1 on which is formed an N-type epitaxial layer 2 properly doped to have the desired Schottky threshold. On this epitaxial N layer is deposited silicon oxide 3 defining a window in which the Schottky contact is desired to be established by means of an appropriate metallization 4. The rear surface of the component is coated with a metallization 5.
Such a structure has a very poor breakdown voltage. Indeed, the equipotential surfaces tend to curve up to rise back to the surface, which results, especially in the curved areas of the equipotential surfaces, in very strong field variations that limit the possible reverse breakdown voltage. To avoid this disadvantage, the structure shown in FIG. 2, in which a peripheral P-type ring 6 is formed at the periphery of the Schottky diode area, is conventionally used in the field of silicon-based components. As a result, the equipotential surfaces must pass in volume under the P regions and thus have a less pronounced curvature. This considerably improves the voltage withstand of the diode. As an example, with silicon of a doping level of 1016 at./cm3, a voltage withstand on the order of 10 V will be obtained with no guard ring and a breakdown voltage on the order of 50 V will be obtained with a guard ring.
However, as previously indicated, such a P-type guard ring cannot be made in a structure formed on a silicon carbide substrate. In this case, the simple structure illustrated in FIG. 1 is not desirable either for the same reasons as in the case of a silicon substrate.
Thus, the present invention aims at providing a Schottky diode structure compatible with simple silicon carbide manufacturing methods and that provides a relatively high breakdown voltage.
To achieve these and other objects, the present invention provides a vertical Schottky diode including an N-type silicon carbide layer of low doping level formed by epitaxy on a silicon carbide substrate of high doping level, in which the periphery of the active area of the diode is coated with a P-type epitaxial silicon carbide layer; a trench crosses the P-type epitaxial layer and penetrates into at least a portion of the height of the N-type epitaxial layer beyond the periphery of the active area; the doping level of the P-type epitaxial layer is chosen so that, for the maximum voltage that the diode is likely to be subjected to, the equipotential surfaces corresponding to approximately xc2xc to xc2xe of the maximum voltage extend up to the trench.
According to an embodiment of the present invention, the doping level of the P-type epitaxial layer is slightly greater than the doping level of the N-type epitaxial layer.
According to an embodiment of the present invention, the distance between the external periphery of the active area and the internal periphery of the trench is on the order of 30 to 60 xcexcm.
The present invention also provides a method for manufacturing a vertical Schottky diode on a heavily-doped N-type silicon carbide substrate, including the steps of forming a lightly-doped N-type epitaxial layer; forming a P-type epitaxial layer; digging a peripheral trench; depositing an insulating layer; forming a central opening crossing the insulating layer, the P-type epitaxial layer and the N-type epitaxial layer; and depositing a metal capable of forming a Schottky barrier with the N-type epitaxial layer.
According to an embodiment of the present invention, the doping level of the P-type epitaxial layer is chosen so that, for the maximum voltage that the diode is likely to be subjected to, the equipotential surfaces corresponding to approximately xc2xc to xc2xe of the maximum voltage extend to the trench.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.